It is highly conventional to utilize films of various oriented polymeric materials for the packaging of foods and like products. Although the necessary and desirable properties depend upon the specific applications for which the films are intended, normally they should exhibit good uniformity, strength, toughness, abrasion and flex-crack resistance, gloss, and heat stability. When formed into closed and sealed packages, such as by heat sealing, the packages should exhibit high burst strength and resistance to cutting, such as by metal closure clips. Also, because of the susceptibility of many food products to deterioration upon exposure to oxygen, it is often imperative that the film employed exhibit good barrier properties.
As is also well known, it is common practice to package meat and other food products in molecularly oriented films that can be shrunk upon exposure to heat. The need for good strength characteristics is particularly acute in such instances because of the vulnerability of the film to damage under the conditions of use.
Generally, the combination of characteristics necessary to provide films that are suited for use in many packaging applications is not afforded in a single layer product, and numerous multiple layer films have been developed or proposed.
Both the film producer and the film user require that oriented films satisfy a multiplicity of requirements. Of primary importance to the film user, in the case of shrink films, is the capability of the film to survive physically intact the processes associated with the film as a package, as it is filled, evacuated, sealed closed and heat shrunk. The film package must also be strong enough to survive the material handling involved in moving the contained product, which may weigh 100 pounds or more, along the distribution system to the next processor or to the user. Thus the package must physically protect the product.
It is often desirable to the film user that the package film serve as a barrier to infusion of gaseous materials from the surrounding environment. Of particular importance is provision of an effective barrier to infusion of oxygen, since oxygen is well known to cause spoilage of food products. The package should also be clear for product appeal purposes.
The film producer requires a film which can be produced competitively while meeting the performance requirements of the user. Thus the film materials should be readily extrudable, and susceptible to orientation, with sufficient leeway in process parameters as to allow for efficient film production. The process should also be susceptible to efficient extended production operations. In the orientation process, the film must be able to withstand the stretching. The orientation temperature should be a temperature which is economically achieved by the producer, and which provides for use of economical shrink processes by the film user.
Conventionally used shrink films are often produced and used as bags and have generally been constructed with ethylene vinyl acetate (EVA) and an oxygen barrier layer such as saran or ethylene vinyl alcohol (EVOH).
Notwithstanding the advantages, shrink film packaging, and particularly shrink bag packaging of meat, is not without its difficulties, many of which are attributable to limitations inherent in the films utilized for such applications. As will be appreciated, the processes of stretching the film, and later shrinking it, subject the film to rather severe conditions, due to the nature of the operations.
It is especially important to appreciate that the film is particularly vulnerable to failure at conditions of operation due to the relatively high temperatures to which it is subjected in the orientation and shrinking processes.
The film must be susceptible to orientation without distortion, or separation of the multiple layers which are normally present in films of this nature. The film must be strong enough, at the orientation temperature to withstand the the stretching without the creation of holes, tears, or nonuniform zones of stretching. In the case of blown tubular film, as in the well-known double bubble process, the film must be capable of physically supporting the stretching bubble during the orientation process. Finally, each of the layers of the film should be susceptible to orientation in the multiple layer film without fracture, separation, or creation of holes.
In shrink packaging use, the film must respond to heat rapidly enough for commercial practicality, and yet must not exhibit such a level of shrink energy as would cause the film to pull apart or delaminate during shrinkage, under its own internal forces. Moreover the shrink related problems may be seriously increased, for example, when a contained cut of meat includes protruding bones and/or significant depressions in its surface.
Prospective films are conveniently subjected to preliminary testing in a laboratory. In one such test, the prospective film is formed into packages by means of heat seals and air is injected into the packages. The recorded parameter is the air pressure that a given package successfully holds without failure of the sidewalls or the seals. Another convenient laboratory test measures the interlayer adhesions at the layer interfaces, by pulling apart the layers and measuring the force required for the pulling. The ultimate goal, of course, is to produce a sealed package, containing the contemplated product; and to have the sealed package, containing the product, retain its integrity as a sealed unit throughout the distribution and sale of the product, to the time of opening by the product user.
In the uses contemplated for oriented films of the invention, the most severe environments normally encountered by the films are those associated with the overall process of a packager, where films may be made into containers, containers are filled with product and are sealed closed, and may also be evacuated and/or heat shrunk. Thus the most significant test is to use the films in the commercial processing operations of a packager, to ascertain the overall reduction in the percentage of packages which leak.
Certain of the available 3-layer films having a saran layer are disclosed in Widiger et al U.S. Pat. No. 4,247,584. Those films provide a good balance of properties. Saran, however, has a number of its own problems. It has a brown color, which is generally undesirable. During extended extruder operation, bits of carbon form from decomposition of the saran in the extruder equipment, and later pass out through the die as undesired inclusions in the film. As a result, the operation must be shut down periodically for die cleaning. Finally, the power required to extrude saran is relatively high. Thus, while saran is accepted as a functional material, alternate barrier material choices are desirable.
It is also suggested in the foregoing Widiger et al patent that a barrier layer of EVOH be used instead of the saran layer, so as to provide the excellent oxygen barrier properties of EVOH while avoiding the disadvantages of saran. And while EVOH does have certain problems associated with its fabrication and use, in certain applications it may be advantageously used.
In further improvement of the technology of oxygen barrier materials, oriented blends of EVOH are disclosed in copending application Ser. No. 290,171 by Schroeder et al, filed Aug. 5, 1981 and in copending application Ser. No. 290,172 by Newsome, also filed Aug. 5, 1981, both of common assignment herewith. The Schroeder et al application discloses oriented films of blends of EVOH and nylon along with plasticizers for the nylon. Newsome '172 discloses oriented films wherein EVOH is blended with any member of a family of polymers.
In another related application, Ser. No. 371,781 by Newsome, filed April 26, 1982, it is proposed that the inclusion of linear low density polyethylene (LLDPE) in at least one of the layers provides a significant decrease in the leaker rate when the film bags are used for shrink packaging of meat.
While the art of oriented multiple layer films is becoming crowded, there still exists a deficiency, as a need for easily produced oriented films which exhibit the afore-mentioned desirable properties. Particularly, there remain certain problems having to do with strength of the film and packages made therefrom and adhesion of the layers to each other at the layer interfaces, in addition to the typical problems with incompatibility of certain polymers with orientation. Thus it is desired to provide a film having improved strength properties imparted generally by creating a film having the desirable characteristics and imparting thereto properties of improved interfacial adhesion, and attendant improvements in the overall strength properties of such films as demonstrated in laboratory tests and in commercial use.